The advent of the energy crisis throughout the world has spurred interest in obtaining alternatives to today's sources of energy. The price of fossil fuel has risen to such a level as to make previous price forbidden alternatives economically competitive. Such an alternative is solar energy. The major obstacle encountered by the prior art in the fields of solar heating and cooling and the provision of domestic hot water was economic, resulting from the high cost of the equipment needed to collect and store solar energy. In addition, there are basic problems inherent in the nature of solar radiation. Solar radiation is relatively low in intensity, rarely exceeding 300 Btu/ft.sup.2 /hr; it is intermittent because the daily variation in direct solar intensity from zero at sunrise to a maximum at noon and back to zero at sunset is inevitable; and it is subject to unpredictable interruptions due to variable weather patterns.
In order to trap the energy from solar radiation a device called a collector is used. This device simplistically contains a gaining surface, a losing surface and means to bring a heat transfer liquid to flow adjacent to the gaining surface. The gaining surface, known as the absorber, is conventionally a black surface upon which solar radiation is converted to heat, and the surface above the black absorber usually parallel thereto, is the losing surface since heat is lost to the outside environment through this surface. Conventionally, the black absorber is also the losing surface since collected heat is lost to the outer portions of the assembly through this surface.
The major objective of solar heat collectors has been to collect as much solar radiation as possible, at the highest attainable temperature for the lowest possible investment in labor and materials. To determine the efficiency of a collector, an efficiency calculation is performed in energy terms. The efficiency of a collector can be described as the heat energy added to the liquid during its passage through the collector, divided by the energy of the impinging sunlight during the time of passage of the fluid. A comparison between the present invention and the prior art reveals a relatively small increase in the absolute value of the efficiency at a particular operating temperature. However, a comparison of the relative efficiencies calculates to be a significant efficiency increase over the prior art. For example at a particular operating temperature, the prior art efficiency is 20% while the present invention efficiency exceeds 24%. This represents an approximately 20% overall increase in efficiency over the prior art.
Up to now, solar heat collectors have suffered from efficiency ratings so low that they require large absorber surface areas to function effectively. The present invention, described below, can collect the same amount of heat as conventional collectors while requiring a smaller amount of collector surface area or, on the other hand, can collect more heat for the same area as prior-art collectors. The large surface area requirement of conventional collectors is costly, and thus effectively inhibits solar radiation from being a viable energy alternative except for a few limited applications and in geographical areas where sunlight is abundant.
In order to maintain cost competiveness the collector should be able to perform efficiently for many years. The collector must, therefore, be designed to accomodate the adverse effects of: the sun's ultraviolet radiation; corrosion or clogging due to acidity, alkalinity or mineral content of the heat transfer liquid; freezing; air-binding; and breakage of glazing due to thermal expansion, hail, or other causes.
Another problem plaguing conventional solar heat collectors is the excessive amount of heat that accumulates when the energy is not being extracted either for storage or for utilization. Since a collector is designed to retain as much heat as possible, an overheating problem is present when the heat transfer liquid is drained or not flowing. This accumulation of excessive heat can cause severe physical damage to the collector. Conventional collectors have either various mechanical devices, such as louvers, designed to reflect or otherwise shield solar radiation from the collector when it is non-operational or conventional collectors are constructed with more expensive heat resistant materials and construction features to withstand temperatures up to 400.degree. F. Other mechanical devices have also been used to reflect or otherwise shield solar radiation from the collector. The limited durability of these mechanical devices is undesirable when compared with the longlife requirements of a solar heat collector. The costs of such devices and associated mechanical problems make this method of dealing with the over-heating problems of solar heat collectors undesirable in general, and in many cases prohibitively expensive.
The background of the art to which this invention relates is described in substantial detail in a publication entitled "Solar Energy Utilization For Heating and Cooling" compiled by John I. Yellott, Arizona State University and referenced as NSF 74-41. This document was prepared under National Science Foundation Grant, GI-39247 and is available from the U.S. Government Printing Office, as stock number 3800-0018.
As described in NSF 74-41, the prior art approaches to solar heat collectors have been numerous, with each possessing deficiencies affecting the overall operation and efficiency of a solar energy system. When the objective of the collector is to heat a liquid the conventional approach has been to attach metal tubes to the absorber, and cause a liquid to flow through the tubes. The tubes are sometimes attached above, below or integral with the absorber surface. The heat transfer liquid absorbs the heat from the absorber surface through the tube surface. The heat transfer liquid may be flowing or not flowing, depending on its operational status.
As a consequence of this design, two major problems must be solved to achieve satisfactory efficiency. In order to transfer the maximum quantity of heat to the heat transfer liquid the absorber and tube material must possess good thermal conductivity characteristics. Additionally, the bond between the tubes and absorber must maximize the transfer of heat with minimal thermal impedance. There are solutions to both problems but the solutions give rise to excessive costs for labor and materials.
Materials most frequently used for absorber plates, in decreasing order of cost and thermal conductivity, are copper, aluminum and steel. The effect of bond conductance has been studied with the conclusion that steel pipes are as good as cooper if the bond conductance between tube and plate is good. Bond conductance can range from a high of 100 Btu/(ft)/.degree.F/hr. for a securely soldered tube to a low of 3.2 for a poorly clamped or badly soldered tube. Bonded plates with integral tubes are among the better alternatives for performance.
Illustrations of various prior art solar heat collectors that have been used to heat water with varying degrees of success are shown in NSF 74-41, FIG. 10 at page 59.12. In all of these prior art configurations, a heat transfer liquid is either flowing in tubes or along prescribed troughs. Designs such as these encourage heat to be lost upwards by convection, reflection or evaporation into the environment. In the tube design the entire absorber surface is exposed and thus radiates heat upward and outwards. Where the trough design is utilized those portions of the absorber exposed will radiate heat upwards and evaporation of the exposed liquid will transfer latent head upward and outward.